Slotted paging mode is a form of discontinuous reception (DRX) operation for a battery-operated mobile radio such as a cellular radiotelephone. The mobile radio is configured for radio communication with one or more remote base stations in a radiotelephone system. In slotted paging mode, when the radiotelephone (also referred to as a mobile station) is in an idle mode (i.e., not engaged in a call), the radiotelephone does not continuously monitor a paging channel but generally remains in a low power state.
Slotted paging mode is critical to the life of the battery of the radiotelephone. The goal of slotted mode operation is to reduce the on time of the radio to a minimum and to power down as much of the radio as possible during sleep periods. In the idle state, the radiotelephone wakes up only during slots preassigned by the radiotelephone system or to process some other condition, such as a user input.
When recovering from a sleep period, the radio must reacquire a radio frequency (RF) link with a base station in the radiotelephone system. Link acquisition and other operations including communication protocols for such a system are defined in an air interface specification. One example of such an specification is Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" (IS-95). IS-95 defines a direct sequence code division multiple access (DS-CDMA or CDMA) radiotelephone system.
To reacquire an RF link, a radiotelephone in a CDMA system must be synchronized with system time, which is the timing maintained by base stations and a network controller in the CDMA system. Timing for the forward link (base station to mobile station) must be maintained by the radiotelephone with the expectation that, when an assigned slot occurs, the radio can wake up quickly, make corrections for timing uncertainties and be ready to acquire and process the paging channel.
Synchronization with the forward link involves alignment of locally generated pseudorandom noise (PN) sequences with PN sequences transmitted by a base station on a pilot channel. The transmitted sequences include a "short PN" sequence which repeats every 26-2/3 ms and a "long PN" sequence which repeats once every 41 days. The radiotelephone contains sequence generators which generate short PN and long PN sequences identical to those used by the base station. The radiotelephone uses a searcher receiver or other mechanism for aligning the short PN sequence with those received from a base station. Once the pilot channel has been acquired, the radiotelephone acquires a synchronization channel and a paging channel. The radiotelephone can then correctly demodulate traffic channels and establish a full duplex link with the base station.
When waking up after a sleep time, the radiotelephone must synchronize with the long PN sequence and short PN sequence. Both the short PN sequence and the frame boundary repeat with reasonable frequency in an IS-95 system. Frame boundaries occur on every third PN roll boundary. A PN roll boundary is defined as the short PN sequence rolling back to its initial value. At the mobile station, the short PN and long PN sequences are generated using a linear sequence generator (LSG). LSGs are described by polynomials and implemented using shift registers and exclusive-or gates. Since the short PN sequence repeats only every 26-2/3 ms, when exiting sleep the LSG can be conveniently stopped at a particular phase in the sequence until the phase correlates with the system PN. The short PN LSG is then restarted, in synchronization with system timing.
The long PN sequence, however, repeats only every 41 days. It is impractical to stop the long PN generator of the radiotelephone (for example, when it is time to sleep), then rapidly clock it to catch up with the system's long PN when it is time to wake up.
Since the short PN sequence and long PN sequence transmitted by the system varying predictably with time, acquiring the PN sequences requires that an accurate time reference be kept at the mobile station during sleep mode. The appropriate PN sequences can be determined for correlation with the system PN sequences upon exit from sleep mode. However, maintaining a highly accurate timing reference requires relatively high power dissipation, which is inconsistent with a low power sleep mode.
In addition to exiting sleep mode during assigned slots, the radiotelephone may also be required to wake up to process or respond to other events occurring asynchronously in the radio. One example of such an event is a user input, such as a keypress of the keypad of the radiotelephone. Response to such an input should be rapid, with no perceptible delay for the user.
Accordingly, there is a need for a method and apparatus for controlling entrance to and exit from slotted paging mode in a mobile station such as a radiotelephone. There is a further need for a low power method and apparatus for keeping accurate time in a mobile station such as a radiotelephone.